Tungsten deposition is commonly used in the manufacture of semiconductors to form metal regions on semiconductor substrates. Tungsten is a refractory metal having properties such as low electrical resistivity, a high melting point, and high resistance to electromigration. The refractory metal properties make tungsten very desirable for many semiconductor structures. Tungsten may be deposited either uniformly over the whole wafer surface or selectively over predetermined portions of the wafer surface depending upon deposition conditions. A common problem in the selective chemical vapor deposition (CVD) of tungsten onto silicon with a tungsten bearing gas such as tungsten hexafluoride, WF.sub.6, is the fact that the chemical reaction which is used to deposit tungsten is self limiting. After tungsten is deposited onto a silicon surface to a certain thickness using tungsten hexafluoride and silicon reduction, hydrogen must be bled into the CVD reaction in order to make tungsten further deposit by a hydrogen reduction type of process. However, unless a proper composition and extremely clean adjacent dielectric surface exists, deposition of tungsten occurs on both the tungsten surface created by the initial part of the process and the adjacent dielectric. The deposition on the adjacent dielectric surface is usually in discrete areas or islands. If the selectivity is poor or the deposition time is long, the islands may grow laterally and eventually form a continuous layer on the dielectric. In such a case, selectivity is totally lost. Adjacent dielectric surfaces are typically either silicon dioxide or silicate glasses. Others have proposed various ways to maintain the silicon oxide surface free of tungsten during the deposition of a thick layer of tungsten on exposed silicon. For example, in U.S. Pat. No. 4,617,087 Iyer et al. teach the use of a plasma in a CVD chamber to maintain silicon dioxide clean enough to obtain a selective tungsten etch. However, additional processing steps are required to create a plasma. Others have treated the dielectric with a wet chemical to prevent tungsten deposition. However, the wet chemical treatment requires an additional process flow step and is not easy to control.